The present invention relates to steam turbines and especially to the nozzles of a steam turbine and to the attachment of the nozzle vanes to the outer ring of the nozzle.
Steam turbines are used to transfer energy from heat, first to kinetic energy of a high velocity jetstream and then to the inner rotating shaft which is then usually employed for electrical power generation. A modern steam turbine typically has a number of stages, each stage having nozzles to change the heat energy to work energy and to direct the coarse of steam onto rotating blades. The fixed nozzles direct the steam onto the blades which form a rotating assembly (or rotor) which is rotated to change the kinetic energy of the jet of steam into shaft horsepower. The shaft or rotor rests on bearings and is closed in a cylindrical casing. The rotor is made to turn smoothly by means of the jets of steam issuing from the nozzles located around the periphery of the turbine cylinder and impinging upon the blades attached to the rotor. Stationary guide vanes function as a nozzle and the steam passes therethrough or is expanded and the velocity used for driving the blade ring. The alternating sequence of stationery nozzles and rotating blades is continued through each stage of the turbine.
The vanes in the nozzles of steam turbines are specifically shaped, and on some designs, typically the last few stages of the low pressure turbine are welded to an inner ring and to an outer ring. These welded vanes typically have profiled tips with trailing edges which edges are so thin that welding the vanes to the outer ring all the way along the profile tip of the vane can cause cracks in the trailing edges. The welding is therefore limited to the thicker portions of the end of the vane. This results in some leakage occurring around the trailing edge because it is not welded to the outer ring and this leads to reduced efficiency. Of greater importance are the vanes in steam turbine stages where the flow has a significant moisture content, where the water tends to collect on the pressure side of the vane and can flow around the unwelded portion of the vane tip and then re-enter the flow from the suction side as relatively large slow moving droplets. Because the droplets are large and slow moving, they have a greater impact. These droplets become a significant factor in the erosion of the turbine components.
The present invention is directed towards a design that prevents or reduces the formation and spill of droplets of moisture and reduces leakage without requiring the welding of the fragile trailing edge of the vanes in the nozzles. There have been in the past a number of methods for fabricating steam turbine diaphragms and rotors as well as nozzle assemblies. It has been suggested to remove portions in the inner ring or the outer ring or both the inner and outer ring for more securely locking bladed elements or nozzle vanes. These prior art devices are primarily directed towards securing blades to shroud ends in both stationary and rotatably turbine elements to more securely hold the blades or vanes to the rotor or the nozzle and are not designed to prevent or reduce the formation or spill of droplets around the trailing edge of nozzle vanes without requiring the welding of the fragile trailing edge of the vanes as in the present invention.